JP,A 10-56413 discloses one known example of a compression ignition internal combustion engine employing a combustion mode in which an air-fuel pre-mixture supplied to a combustion chamber is self-ignited with the compressive operation of a piston. The compression ignition internal combustion engine can realize the engine operation in an ultra lean range (at an air-fuel ratio of 80 or more), which has not been realized with gasoline and diesel engines in the past. Also, the compression ignition internal combustion engine can realize combustion that is ignitable at a lower flame temperature and by a uniform air-fuel mixture. It is hence possible to achieve a substantial reduction of both NOx and soot at the same time.
Generally, when an air-fuel pre-mixture is compressed and reaches a certain temperature, the so-called “low-temperature oxidation reaction” begins with dehydrogenation of hydrocarbon, i.e., fuel, which serves as a start reaction. The progress of the low-temperature oxidation reaction causes self-ignition via an elementary reaction called a blue flame. Since this self-ignition occurs simultaneously at multiple points in the air-fuel mixture, a period of resulting combustion is much shorter when looking at the whole of a combustion chamber than a period of combustion caused by spark ignition in a conventional gasoline engine or a period of spray combustion caused in a conventional diesel engine. That feature is effective in suppressing generation of NOx which depends on the flame temperature and a duration thereof, and hence functions as a factor for realizing a reduction of NOx in the compression ignition internal combustion engine.
However, the known compression ignition internal combustion engine has problems as follows. In the known engine, when the engine is operated with compression ignition, the pressure and temperature of a mixture of fuel and air are increased so as to promote chemical reactions and to cause self-ignition with the compressive operation of a piston after closing of an intake valve. Accordingly, if an adiabatic compression condition, for example, of the air-fuel mixture changes with, e.g., fluctuations of the engine operating condition, the ignition timing is varied and a misfire is caused, thus resulting in a problem that it is difficult to perform the normal engine operation. Another problem has been experienced in that, because the self-ignition of the air-fuel mixture is affected by not only the temperature and pressure in the combustion chamber, but also by an air-fuel ratio of the air-fuel mixture, the ignition timing is also changed depending on a fuel diffusion state in the combustion chamber, thus resulting in a cycle variation and a torque variation among cylinders.
In view of those problems, as disclosed in, e.g., JP,A 10-196424, there is proposed a compression ignition internal combustion engine in which a control piston serving as auxiliary compression means is disposed in the combustion chamber and the control piston is operated near the top dead center to reduce the capacity of the combustion chamber and to transiently raise the temperature of the air-fuel mixture, thereby controlling the ignition timing of the air-fuel mixture.